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Biomedical subjects

J E Farrell

Publications and source records attributed to J E Farrell.

18 recordsLinked to original sources
PubMed1990

The visible persistence of stimuli in stroboscopic motion.

This paper reports an improved paradigm to measure visible persistence. The stimulus is a pair of lines stroboscopically displayed in successive positions moving in opposite directions. The subjects' judgement of simultaneous appearance of all the presented lines is used to estimate visible persistence. This paradigm permitted independent manipulation of spatial and temporal stimulus separations in linear motion. The resulting estimates of visible persistence increase with spatial separation up to 0.24 deg of visual angle and approaches a maximum value at larger spatial separations. The results are consistent with the existence of a hypothetical visual gain mechanism that operates over small retinal distances to effectively decrease persistence duration with decreasing spatial separation.

Afterimage↗
PubMed1990

Equating character-identification performance across the visual field.

The effect of retinal eccentricity on stimulus identification was investigated by measuring observers' ability to identify digits of various sizes presented briefly at different locations in their visual fields. The stimuli were either presented one at a time or presented in pairs with each member of the pair at a different eccentricity and having a different size. Stimulus-response confusion matrices were used to calculate the information transmitted by each character as a function of stimulus size and retinal eccentricity. Linearly scaling stimulus size with retinal eccentricity yields equal information transmission across each subject's visual field. There was no difference between the amounts of information transmitted when the targets were presented alone and when they were transmitted in pairs. For small target sets, therefore, the amount of information transmitted from the screen to the observer can be approximated as the sum of the information at the separate locations.

Humans↗
PubMed1990

Discriminability metric based on human contrast sensitivity.

We evaluated a metric for predicting the discriminability of different digitized versions of alphanumeric characters. The metric is based on the assumption that there exists a visual filter such that discriminability is monotonic with the contrast energy in the visually filtered difference between stimuli. To test this hypothesis, we presented two same or different digital versions of a master character and asked subjects to indicate whether the characters were the same or different in a forced-choice procedure with feedback. The filtered contrast energy difference was calculated by convolving the difference between stimulus pairs with filters derived from published human contrast sensitivity functions, following an initial nonlinear transformation of stimulus intensity, and summing the squared result. For some types of stimulus difference, such as contrast quantization errors and Gaussian blurring, performance on discrimination tasks is monotonically related to the contrast energy of the filtered difference vector. The results are consistent with the hypothesis that there exists a single psychometric function that can predict the discriminability of different digitized versions of characters when displayed on various devices.

Contrast Sensitivity↗
PubMed1990

Class II glass ionomer/silver cermet restorations and their effect on interproximal growth of mutans streptococci.

The release of fluoride from glass ionomer materials is one of the most important features of this newly implemented material, and the remineralization effects of this phenomenon have been documented (Hicks and Silverstone 1986). This paper examines the effects of glass ionomer/silver cermet restorations on the plaque levels of interproximal mutans streptococci. Fifteen patients with Class II lesions in primary molars were selected for study. Interproximal plaque samples were obtained from each of the lesion sites and from one caries-free site approximal to a primary molar. One lesion was restored with composite resin to serve as a treated control to the glass ionomer/silver cermet (Ketac Silver, ESPE/Premier Sales Corp., Norristown, Pennsylvania) test site. A sound (unaltered) interproximal site served as the untreated control site. Plaque samples were collected before and at one week, one month, and three months post-treatment. Samples were serially diluted to enable colony counts of mutans streptococci. One week post-treatment counts showed that the glass ionomer/silver cermet restorations significantly reduced (P less than 0.05) the approximal plaque levels of mutans streptococci. Conversely, the untreated and treated control sites did not exhibit reductions in approximal plaque levels of mutans streptococci. These results indicate that glass ionomer restorations may be inhibitory to the growth of mutans streptococci in dental plaque approximal to this restorative material in the primary dentition.

Cermet Cements↗
PubMed1984

Visible persistence of moving objects.

A single line was presented in a succession of orientations, each orientation separated by a fixed angle and by a fixed interval of time, and subjects reported the number of successive lines that appeared to rotate together. The perceived number of rotating lines increased linearly with the rate of stimulus presentation, with a slope that was proportional to the spatial separation. The linear functions obtained in this first experiment predicted the results of a second experiment in which subjects adjusted the spatial and temporal variables to a discrimination threshold for seeing two rotating lines. If the slope of the linear functions is considered to be an estimate of the duration of visible persistence, then these results suggest that the visible persistence of a briefly presented stimulus increases with the distance separating that stimulus from other stimuli.

Afterimage↗
PubMed1983

Visual apparent movement: transformations of size and orientation.

Sequential alternation between same-shaped stimuli differing in size (size ratio s) and orientation (angular difference v) produced a visual illusion of translation in depth and concurrent rotation. The minimum stimulus-onset asynchrony required for the appearance of a rigidly moving object was approximately a linearly increasing function of (s-1)/(s+1) for simple translation in depth and a linearly increasing function of v for simple rotation. The extrapolated zero intercept was lower for translation than for rotation, but estimated transformation times were additive in combined transformations. The results suggest that (a) the processes of apparent translation in depth and apparent rotation are individually sequential-additive in structure, and (b) apparent translations and rotations are combined by fine-grained alternation of steps of apparent translation and steps of apparent rotation. Similar principles account for recent data on imagined spatial transformations of visual size and orientation.

Depth Perception↗
PubMed1982

Velocity constraints on apparent rotational movement.

The visual illusion of apparent rigid rotation was produced by sequential alternation of two views of the same object in different orientations. The minimum stimulus-onset asynchrony required for the appearance of rigid rotation was a linearly increasing function of the angular difference in orientation between the two views. Variation in the size of the object affected the zero-intercept of the function, but the slope was virtually constant. The slope invariance suggests that the appearance of rigid rotation is constrained by an upper bound on the apparent angular velocity of the object as a whole, rather than a bound on the linear velocity of its parts.

Humans↗
PubMed1981

Shape, orientation, and apparent rotational motion.

Apparent rotational motion was investigated in polygonal shapes ranging in rotational symmetry from random to self-identical under 180 degrees rotation. Observers adjusted the rate of alternation between two computer-displayed orientations of any given polygon to determine the point of breakdown of perceived rigid rotation between those two orientations. For asymmetric polygons, the minimum stimulus-onset asynchrony yielding apparent rigid rotation increased approximately linearly with orientational disparity, as anticipated on the basis of the extension of Korte's third law to rotational motion by Shepard and Judd. For nearly symmetric polygons, however, the critical time increased markedly as the disparities approached 180 degrees, owing to the availability of a shorter, nonrigid rotation in the opposite direction. The results demonstrate the existence of competing mental tendencies to preserve the rigid structure of an object and to traverse a minimum transformational path.

Discrimination, Psychological↗